Method of producing steel sheets for porcelain enameling having improved enamel adhesion property

ABSTRACT

Activation of steel sheet surface by subjecting to a pickling treatment after recrystallization annealing and before forming in the production steps of steel sheets for coating porcelain enameling. Continuously cast slabs are used as a starting steel, and the resulting sheet realizes an improved adhesion property, to porcelain glaze without causing fishscale, blistering and pinhole defects and the like even when using a glaze not requiring pickling with sulfuric acid, Ni flashing or the like as an enameling glaze.

TECHNICAL FIELD

This invention relates to a method of producing steel sheets forporcelain enameling having an improved enamel adhesion property evenwhen using an enameling glaze without conducting sulfuric acid pickling,Ni flashing or the like as a pretreatment.

BACKGROUND ART

Porcelain enamels are non-combustible materials obtained by baking avitreous substance onto a surface of a steel sheet, and have variousproperties such as heat resistance, weather resistance, resistance tochemicals, water resistance, stain resistance and the like as well as abeautiful surface appearance. As to the steel sheet material used forporcelain enameling, it is required to have firing strain property,fishscale resistance, enamel adhesion property, resistance to blisteringand pinhole defects and the like as well as press formability inaccordance with use or purpose, among which properties the enameladhesion property is particularly important together with the fishscaleresistance.

Heretofore, decarburization capped steels have been used as a steelsheet for porcelain enameling satisfying the above properties. Suchdecarburization capped steels are produced by subjecting a slab afteringot making-blooming step to hot rolling and cold rolling and thensubjecting the resulting sheet to decarburization and denitrationthrough open-coil annealing process. However, continuous operations suchas continuous casting, continuous annealing and the like are used forenergy-saving and cost reduction in the existing steel-making process,so that the production method and steps using the above decarburizationcapped steel become a past process.

When steel sheets having excellent fishscale resistance, enamel adhesionproperty and press formability are produced by the above continuouscasting method, extra-low carbon steels having a C content of not morethan 0.005 wt % (hereinafter shown by % simply) are used as a startingmaterial and combined with Ti or B as disclosed, for example, inJapanese Patent laid open No. 61-276958 and Japanese Patent ApplicationPublication No. 54-3446. The thus continuously cast steel sheets forporcelain enameling are almost treated in such a manner that they arepickled with sulfuric acid in a weight reduction through pickling of20-100 g/m² so as to form uneven portions of about 1 μm on the surfaceof the steel sheet or further subjected to Ni flashing to precipitatemetallic Ni on the top of the convex portion formed through the picklingin an amount of 0.1-5.0 g/m², and then subjected to a single enamelingin which an undercoat glaze (ground coat) containing about 0.5% of eachof NiO and CoO as an oxide for promotion of adhesion property is appliedand fired or a double enameling may be applied as in FIG. 3 in which atopcoat glaze (cover coat) is applied after the application of ground.These have a good adhesion property {PEI adhesion index>80%: PEI[adhesion test method (ASTM: C313-59) recommended by P.E.I. (EnamelingAssociate in USA)]}.

In the existing enameling makers, an enameling method is providedwherein only alkali degreasing is conducted as a pretreating operationand an undercoat glaze (ground coat) containing about 1.0% of each ofNiO and CoO as an oxide for the promotion of adhesion property isdirectly applied and fired without pickling with sulfuric acid and Niflashing, as shown in FIG. 2. That is, this method is advantageous tothe enameling maker because the pretreating step is omitted and thedisposal of waste liquid from the pretreating liquid is useless to bringabout the large reduction of the cost.

However, when adopting the enameling method using this type of theglaze, the continuously cast steel sheets for porcelain enameling havestill a problem that the adhesion property is considerably poor ascompared with the decarburization capped steel. Particularly, even whenthe good adhesion property is exhibited at the stage of single undercoatenameling, the remarkable degradation of the adhesion property is causedat the stage of double enameling.

DISCLOSURE OF INVENTION

This invention is to advantageously solve the aforementioned problem andto provide a method of advantageously producing steel sheets forenameling having an adhesion property equal to or more than that of theconventional decarburization capped steel without damaging theproperties such as press formability, fishscale resistance and the likeeven using only the aforementioned alkali degreasing.

That is, the invention lies in a method of producing cold rolled steelsheets for enameling having an improved adhesion property, whichcomprises subjecting a continuously cast slab having a given chemicalcomposition to hot rolling and cold rolling according to the usualmanner and then subjecting the resulting sheet to a pickling treatmentafter recrystallization annealing and before forming.

In the invention, it is preferable that a weight reduction through thepickling treatment is obtained within a range of 0.1-20.0 g/m².

Moreover, a flow chart for the production steps of enameled sheetaccording to the invention is shown in FIG. 1.

The experimental results succeeding in the invention will be describedbelow.

EXPERIMENT I

Four kinds of steel sheets for enameling after cold rolling andannealing having chemical compositions as shown in Table 1 and producedby production steps shown in the same table, which steels A-C arecontinuously cast steels and steel D is a decarburization capped steelin Table 1) were pickled in an aqueous solution of 5% HCl at 50° C. soas to have a weight reduction through pickling of 0.5 g/m², subjected toa skin pass rolling at a reduction of 0.5%, enameled with a commerciallyavailable undercoat glaze containing 1.3% of NiO and 0.8% of CoO at athickness of 100 μm, dried at 160° C. for 10 minutes, and then fired at830° C. for 3 minutes. Thereafter, a commercially available topcoatglaze was applied at a thickness of 100 μm thereto, which was dried at160° C. for 10 minutes and fired at 800° C. for 3 minutes.

The adhesion property was measured with respect to the thus obtainedenameled sheets to obtain results as shown in Table 2.

Moreover, the adhesion property was measured with respect to steelsheets not subjected to pickling with HCl after the firing for thecomparison and the measured results are also shown in Table 2.

                                      TABLE 1                                     __________________________________________________________________________    Chemical Composition (%)                                                      Steel                                                                            C   Si Mn P  S  Al N   Cu O   Ti B   Se Production steps                   __________________________________________________________________________    A  0.0019                                                                            0.010                                                                            0.20                                                                             0.009                                                                            0.026                                                                            0.042                                                                            0.0076                                                                            0.032                                                                            0.0031                                                                            0.098                                                                            --  0.005                                                                            CC*.sup.1) -CAL*.sup.2)            B  0.0021                                                                            0.008                                                                            0.16                                                                             0.010                                                                            0.010                                                                            0.001                                                                            0.0068                                                                            0.028                                                                            0.0190                                                                            -- 0.0126                                                                            -- CC-BA*.sup.3)                      C  0.0009                                                                            0.008                                                                            0.15                                                                             0.008                                                                            0.009                                                                            0.001                                                                            0.0081                                                                            0.031                                                                            0.0157                                                                            -- 0.0130                                                                            -- CC-CAL                             D  0.0017                                                                            0.010                                                                            0.25                                                                             0.007                                                                            0.009                                                                            0.001                                                                            0.0010                                                                            0.031                                                                            0.0512                                                                            -- --  -- Capped*.sup.4) -OCA*.sup.5)        __________________________________________________________________________     *.sup.1) CC: continuously cast steel                                          *.sup.2) CAL: continous annealing                                             *.sup.3) BA: box annealing                                                    *.sup.4) Capped: capped steel                                                 *.sup.5) OCA: decarburization denitration annealing (Opencoil annealing) 

                  TABLE 2                                                         ______________________________________                                        PEI adhesion index (%)                                                        Single undercoat                                                              enameling      Double enameling                                                    no                no                                                          pickling pickling pickling                                                                             pickling                                        Steel                                                                              with HCl with HCl with HCl                                                                             with HCl*                                                                             Remarks                                 ______________________________________                                        A    92       100      55     90      CC-CAL                                  B    85       100      60     96      CC-BA                                   C    87       100      50     93      CC-CAL                                  D    100      100      85     90      Capped-OCA                              ______________________________________                                         *Invention method                                                        

As seen from the above tables, when all steels are subjected to thepickling with HCl after the annealing, the enamel adhesion property isgood.

Then, the steel sheet A was subjected to 1 annealing pickling cupping ata punching diameter of 100 mm and a drawing ratio of 1.5, 2annealing--skin pass rolling at a reduction of 1.0%--pickling--cupping,or 3 annealing--skin pass rolling--cupping--pickling, which was furthersubjected to the aforementioned enameling. The fishscale resistance,state of generating blistering and pinhole defects and resistance tosecondary forming brittleness were measured with respect to the thusobtained enameled steel sheets to obtain results as shown in Table 3.

The fishscale resistance was evaluated by sample numbers generatingfishscales when 10 enameled samples were left to stand in a thermostaticchamber of 160° C. for 10 hours.

The blistering and pinhole defects were judged by the presence orabsence of the defect when visually observing the appearance of thesheet after the firing.

The resistance to secondary forming brittleness was evaluated by samplenumber generating breakage when the sample after the pickling wasimmersed in a solution of (alcohol+dry ice) at -60° C. for 10 minutesand then a weight of 5 kg was fallen down thereonto from a height of 80cm.

                  TABLE 3                                                         ______________________________________                                                                     Resistance                                                           Blistering                                                                             to                                                                   and      secondary                                        Timing of  Fishscale                                                                              pinhole  forming                                          picking    resistance                                                                             defects  brittleness                                                                           Remarks                                  ______________________________________                                        1    just after                                                                              0        0      0       acceptable                                  annealing                         example                                2    just after                                                                              0        0      0       acceptable                                  skin-pass                         example                                     rolling                                                                  3    after press                                                                             1        3      1       com-                                        forming                           parative                                                                      example                                ______________________________________                                    

As seen from the above table, the sample subjected to the picklingbefore the press forming shows less occurrence of each of fishscale,blistering and pinhole defect and secondary forming breakage as comparedwith the sample subjected to the pickling after the press forming.

EXPERIMENT II

Among steels shown in Table 1, the steel A was pickled with an aqueoussolution of 10% HCl at 70° C. so as to have a weight reduction of0.05-33.05 g/m², and then subjected to an enameling in the same manneras in Experiment I.

The adhesion property of the thus obtained enameled sheet was measuredto obtain results as shown by a relation to weight reduction throughpickling in FIG. 4.

As seen from this figure, the good adhesion property was particularlyobtained when the weight reduction through pickling was within a rangeof 0.1-20.0 g/m².

According to the invention, the reason why the adhesion property isimproved by the pickling after the annealing is not yet clearlyelucidated, but is considered as follows.

In case of the continuously cast steel sheet for porcelain enameling,elements such as Ti, B and the like are added for the purpose ofensuring the fishscale resistance and the press formability. However,these elements are liable to form an oxide, so that an oxide film isformed on the surface of the steel sheet during the firing. In the steelsheet having such an oxide film, it is considered that since thereaction between the steel sheet and the glaze (dissolving of Fe intothe enameled layer) is insufficient in the firing after the applicationof an enameling glaze, the good adhesion property is not obtained. Infact, it has been confirmed even from the inventors' experiments thatwhen the enamel is thin in the single enameling, the adhesion is barelymaintained, but when the enamel becomes thick in the double enameling,peeling of the enameled layer occurs.

On the other hand, it is considered that when light pickling is appliedbefore the forming, the surface of the steel sheet is activated topromote the reaction of steel sheet--enamel and hence the bonding forceis strengthened to improve the adhesion property.

Moreover, when the pickling treatment is carried out after pressforming, fishscale, blistering and pinhole defects and secondary formingbreakage are apt to be caused. This is considered due to the fact thatthe strain quantity applied by the press forming is fairly larger thanthat by the skin pass rolling so that hydrogen contained in a picklingsolution is liable to penetrate into the steel sheet in the subsequentpickling and brings about the following:

i) The degradation of the resistance to secondary forming brittleness ispromoted;

ii) When strain is released in the firing of enamel, hydrogen isdischarged from the steel sheet into the enameled layer, whereby bubblesare formed in the enameled layer to generate the blistering and pinholedefects;

iii) Therefore, fishscaling is caused.

And also, it is considered that when excessive pickling treatment iscarried out in such a manner that the weight reduction through picklingexceeds a certain value, the adhesion property is reversely degraded dueto the adhesion of the pickled product (smut) to the steel sheetsurface.

Then, each of the production steps will be described concretely.

Chemical Composition of Steel Sheet

The invention is fundamentally applied to continuously cast steel sheetsfor porcelain enameling, and may be applied to decarburization cappedsteels. And also, the chemical composition is not particularly critical,and steel sheets for porcelain enameling having a chemical compositionas shown in JIS G3133 may be used. Moreover, as to C amount, it ispreferable for the amount of C to be: not more than 0.008%. Since C isan interstitial solid soluting element, when the amount exceeds 0.008%,not only the considerable hardening of the material is caused, but alsoCO₂ gas is produced in the firing of enamel to raise a risk of causingthe blistering and pinhole defects.

Hot Rolling, Cold Rolling Conditions

According to the invention, the hot rolling is not particularlyrestricted. Even when the hot rolling is terminated at a temperature ofnot lower than usual Ar₃ transformation point, or even when thelow-temperature finishing of not higher than Ar₃ transformation point iscarried out, the enameling properties are not affected too much. Whenthe mechanical properties of the steel sheet are important, thefinishing temperature for the hot rolling is desirable to be not lowerthan Ar₃ transformation point. Furthermore, when the mechanicalproperties are held in the coiling, the temperature is favorable to behigher, particularly not lower than 500° C. However, the scale layerbecomes thicker at a coiling temperature of not lower than 700° C. tolower the descaling property (pickling property), so that the upperlimit is desirable to be about 700° C.

And also, the cold rolling conditions are not particularly restricted inthe invention. When producing cold rolled steel sheets having goodmechanical properties, particularly drawability (r-value), the coldrolling reduction is preferable to be not less than 70%.

Annealing Conditions

According to the invention, the recrystallization annealing is notparticularly restricted. That is, even when adopting anyone of boxannealing, open-coil annealing and continuous annealing, the enamelingproperties such as adhesion property and the like as well as mechanicalproperties are not badly affected. However, the annealing temperature ispreferred to be a range of not lower than the recrystallizationtemperature but not higher than the Ac₃ transformation point. Becausewhen the recrystallization is incomplete, the formability isconsiderably degraded and the press cracking is apt to be caused in thepress forming or the like, while when the temperature exceeds the Ac₃transformation point, the recrystallization texture is randomized tolower the drawability.

Pickling:

It is a most significant feature to conduct the pickling after theannealing and before the forming. When the sheet is subjected to such apickling treatment, the improved adhesion property can be obtained evenin the single undercoat enameling or even in double enameling which isusually considered to bring about remarkable degradation of the adhesionproperty. Such a pickling treatment is enough to activate the surface ofthe steel sheet so as to provide a reaction layer of enamelingadvantageous for the adhesion property as mentioned above, so that thetemperature and concentration of the pickling solution and the picklingtime are not particularly restricted. Furthermore, the pickling solutionmay be aqueous solution of HCl or H₂ SO₄, and the kind of the acid isnot particularly restricted.

The reason why the pickling treatment is carried out in the steel sheetmaker prior to the enameling inclusive of the forming in the enamelingmaker is due to the fact that since the strain quantity applied by theforming is considerably larger that that by the skin pass rolling, ifpickling is carried out after the forming, hydrogen included in thepickling solution is apt to penetrate into the steel sheet and hencevarious troubles are apt to be caused by this hydrogen as previouslymentioned.

In such a pickling step, when the weight reduction through pickling isless than 0.1 g/m², the pickling effect is less, while when it exceeds20.0 g/m², the amount of the pickled product adhered to the steel sheetsurface becomes so large as to rather degrade the adhesion property, sothat the weight reduction through pickling is preferable to be 0.1-20.0g/m².

Skin Pass Rolling

In the invention, skin pass rolling does not particularly affect theadhesion property, but is advantageous for correcting the shape of thesteel sheet. As the treating conditions, the usual reduction, tensionand passing speed are sufficient.

The cold rolled steel sheets for porcelain enameling produced by theaforementioned production steps according to the invention have anadhesion property equal to or more than that of the conventionalcarburization capped steel even in case of the continuously cast steelsheets.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart showing production steps of enameled sheetaccording to the invention;

FIG. 2 is a flow chart showing production steps of conventional enameledsheet omitting pickling with sulfuric acid and Ni flashing steps;

FIG. 3 is a flow chart showing production steps of conventional enameledsheet inclusive of pickling with sulfuric acid and Ni flashing steps;and

FIG. 4 is a graph showing a relation between weight reduction and PEIadhesion property.

BEST MODE FOR CARRYING OUT THE INVENTION

Each of continuously cast slabs (steels E-H) and bloomed slab (steel I)having a chemical composition shown in Table 4 was held by heating at1200° C. for 3 hours and rough rolled into a sheet bar of 30 mm inthickness, which was subjected to a tandem hot rolling at a finishtemperature of 880° C. to obtain a hot rolled sheet of 3.5 mm inthickness and then coiled at 620° C. After the descaling, the sheet wascold rolled to a thickness of 0.8 mm through 4-stand cold rolling milland recrystallization annealed by passing through a continuous annealingline under heat cycles of heating rate: 10° C./s, soaking temperature:830° C., soaking time: 2 seconds to 5 minutes and cooling rate: 15° C./s(Moreover, the steel I was recrystallization annealed by open-coilannealing of decarburization and denitration at 670° C. for 10 hours).

Then, the sheet was subjected to a pickling treatment with a picklingsolution of HCl under conditions of concentration: 10%, temperature: 60°C. and immersing time: 15 seconds to 10 minutes. On the other hand, thesheet not subjected to the pickling was subjected to skin pass rollingat a reduction of 0.4-1.3%.

Thereafter, these sheets were subjected to a pretreatment for enameling(only alkali degreasing), enameled with a commercially availableundercoat glaze containing a great amount of NiO and CoO at a thicknessof 100 μm, and fired at 840° C. for 3 minutes according to the stepsshown in FIG. 1. Furthermore, these enameled sheets were enameled with acommercially available topcoat glaze at a thickness of 100 μm, and firedat 800° C. for 3 minutes.

The enamel adhesion property was measured with respect to the thusobtained product sheets through PEI adhesion test [adhesion test method(ASTM: C13-59) recommended by PEI (Enameling Associate in USA)] toobtain results as shown in Table 5.

As to steel sheets of Run Nos. 16-35, there were provided ten samplessubjected to 1 annealing--pickling--cupping at a punching diameter of100 mm and a drawing ratio of 1.5, 2 annealing--skin pass rolling--pickling--cupping, or 3 annealing--skin passrolling--cupping--pickling. These cupped samples were enameled and thenthe cup number generating the blistering and pinhole defects wasmeasured by visual observation to obtain results as shown in Table 5.

Moreover, the samples of Run Nos. 34, 35 not subjected to skin passrolling were similarly examined to obtain results as shown in Table 5.

                                      TABLE 4                                     __________________________________________________________________________    Chemical Composition (%)                                                      Steel                                                                            C   Si Mn P  S  Al N   Cu O   Ti B   Se Production steps                   __________________________________________________________________________    E  0.0021                                                                            0.008                                                                            0.15                                                                             0.006                                                                            0.009                                                                            0.041                                                                            0.0072                                                                            0.031                                                                            0.0033                                                                            0.099                                                                            --  0.005                                                                            CC*.sup.1) -CAL*.sup.2)            F  0.0017                                                                            0.011                                                                            0.20                                                                             0.011                                                                            0.010                                                                            0.001                                                                            0.0067                                                                            0.029                                                                            0.0150                                                                            -- 0.0130                                                                            -- CC-CAL                             G  0.0015                                                                            0.004                                                                            0.25                                                                             0.009                                                                            0.017                                                                            0.001                                                                            0.0072                                                                            0.023                                                                            0.0133                                                                            -- 0.0127                                                                            -- CC-BA*.sup.3)                      H  0.0022                                                                            0.004                                                                            0.03                                                                             0.005                                                                            0.008                                                                            0.002                                                                            0.0081                                                                            0.030                                                                            0.0028                                                                            0.120                                                                            --  0.001                                                                            CC-BA                              I  0.0012                                                                            0.009                                                                            0.24                                                                             0.006                                                                            0.010                                                                            0.001                                                                            0.0008                                                                            0.027                                                                            0.0527                                                                            -- --  -- Capped*.sup.4) -OCA*.sup.5)        __________________________________________________________________________     *.sup.1) CC: continuously cast steel                                          *.sup.2) CAL: continuous annealing                                            *.sup.3) BA: box annealing                                                    *.sup.4) Capped: capped steel                                                 *.sup.5) OCA: decarburization denitration annealing (Opencoil annealing) 

                                      TABLE 5(a)                                  __________________________________________________________________________           Presence or                  Cup number causing blistering and                absence of                                                                           Weight    PEI adhesion                                                                              pinhole defects (cups)                           pickling with                                                                        reduction                                                                          Skin-pass                                                                          property (%)       pickled sheet                                                                        pickled                            HCl after re-                                                                        through                                                                            rolling                                                                            Single      pickled sheet                                                                        just after                                                                           sheet                       Run    crystallization                                                                      pickling                                                                           reduction                                                                          undercoat                                                                           Double                                                                              just after                                                                           skin-pass                                                                            after                       No. Steel                                                                            annealing                                                                            (g/m.sup.2)                                                                        (%)  enameling                                                                           enameling                                                                           annealing                                                                            rolling                                                                              forming                                                                            Remarks                __________________________________________________________________________    1   E  presence                                                                             0.35 0.5  100   100   --     --     --   acceptable                                                                    example                2   "  "      1.50 0.8  100   100   --     --     --   acceptable                                                                    example                3   "  "      0.15 0.5  95    90    --     --     --   acceptable                                                                    example                4   "  "      0.93 1.0  100   100   --     --     --   acceptable                                                                    example                5   "  absence                                                                              --   0.8  85    40    --     --     --   comparative                                                                   example                6   F  presence                                                                             0.11 0.6  100   92    --     --     --   acceptable                                                                    example                7   "  "      0.31 0.8  100   93    --     --     --   acceptable                                                                    example                8   "  absence                                                                              --   0.7  80    44    --     --     --   comparative                                                                   example                9   G  presence                                                                             0.66 0.4  100   90    --     --     --   acceptable                                                                    example                10  "  "      1.58 0.9  95    94    --     --     --   acceptable                                                                    example                11  "  "      1.00 0.5  100   100   --     --     --   acceptable                                                                    example                12  "  absence                                                                              --   0.6  73    50    --     --     --   comparative                                                                   example                13  H  presence                                                                             0.41 0.5  95    88    --     --     --   acceptable                                                                    example                14  "  "      2.05 1.3  100   100   --     --     --   acceptable                                                                    example                15  "  absence                                                                              --   0.5  90    70    --     --     --   comparative                                                                   example                16  E  presence                                                                             0.57 0.6  100   100   --     --     --   acceptable                                                                    example                17  I  absence                                                                              --   0.8  98    92    --     --     --   comparative                                                                   example                __________________________________________________________________________

                                      TABLE 5(b)                                  __________________________________________________________________________           Presence or                  Cup number causing blistering and                absence of                                                                           Weight    PEI adhesion                                                                              pinhole defects (cups)                           pickling with                                                                        reduction                                                                          Skin-pass                                                                          property (%)       pickled sheet                                                                        pickled                            HCl after re-                                                                        through                                                                            rolling                                                                            Single      pickled sheet                                                                        just after                                                                           sheet                       Run    crystallization                                                                      pickling                                                                           reduction                                                                          undercoat                                                                           Double                                                                              just after                                                                           skin-pass                                                                            after                       No. Steel                                                                            annealing                                                                            (g/m.sup.2)                                                                        (%)  enameling                                                                           enameling                                                                           annealing                                                                            rolling                                                                              forming                                                                            Remarks                __________________________________________________________________________    18  I  presence                                                                              0.24                                                                              0.7  100   100   --     --     --   comparative                                                                   example                19  E  "      10.16                                                                              0.5  100   100   0      --     --   acceptable                                                                    example                20  "  "      "    "    "     "     --     0      --   acceptable                                                                    example                21  "  "      "    "    "     "     --     --     2    comparative                                                                   example                22  "  "       0.05                                                                              "     90    35   0      --     --   comparative                                                                   example                23  F  "      15.01                                                                              "    100   100   0      --     --   acceptable                                                                    example                24  "  "      "    "    "     "     --     0      --   acceptable                                                                    example                25  "  "      "    "    "     "     --     --     2    comparative                                                                   example                26  "  "      25.16                                                                              "     92    50   0      --     --   comparative                                                                   example                27  G  "      19.02                                                                              "    100   100   0      --     --   acceptable                                                                    example                28  "  "      "    "    "     "     --     0      --   acceptable                                                                    example                29  "  "      "    "    "     "     --     --     3    comparative                                                                   example                30  "  "      65.32                                                                              "    "      70   0      --     --   comparative                                                                   example                31  H  "      13.04                                                                              "    "     100   0      --     --   acceptable                                                                    example                32  "  "      "    "    "     "     --     0      --   acceptable                                                                    example                33  "  "      "    "    "     "     --     --     2    comparative                                                                   example                34  "  "       9.26                                                                              0    "     "     0      --     --   acceptable                                                                    example                35  "  "      "    "    "     "     --     --     1    comparative                                                                   example                __________________________________________________________________________

As seen from the above tables, all kinds of the steel sheets pickledwith HCl after the recrystallization annealing exhibited good adhesionproperty as compared with the sheets not subjected to pickling even inthe single undercoat enameling or double enameling. Furthermore, theblistering and pinhole defects occurred in the sheets pickled after thepress forming, while there was caused no occurrence of the blisteringand pinhole defects in the sheets pickled before the press formingaccording to the invention.

Moreover, when the pickling with HCl was not carried out, the adhesionproperty was good after the single undercoat enameling, but the adhesionproperty tended to rapidly lower in case of the double enameling.

INDUSTRIAL APPLICABILITY

According to the invention, even when the starting steel is acontinuously cast steel, there can be obtained steel sheets forenameling having an adhesion property equal to or more than that of thedecarburization capped steel and less enameling defects. And also, theinvention largely contributes to energysaving and reduction of cost inthe manufacture of the steel sheet.

Furthermore, since the steel sheets obtained according to the inventioncan continuously be pickled at a side of steel makers, the conventionalbatch type pretreating steps such as pickling, Ni flashing and the likefor small size products after the forming conducted at a side of theenameling makers can completely be omitted, so that the production costcan largely be reduced by decreasing the production steps in theenameling maker.

We claim:
 1. A method of producing cold rolled steel sheets havingimproved properties for enameling and having an improved enamel adhesionproperty, which method comprises the steps of subjecting a continuouslycast slab of carbon steel to hot rolling and cold rolling to make steelsheet according to the usual manner, subjecting the resulting sheet torecrystallization annealing, subjecting said sheet to a picklingtreatment after said recrystallization annealing, and thereafter formingsaid sheet into a shape and coating said shape with porcelain enamel. 2.A method of producing steel sheets for porcelain enameling according toclaim 1, wherein the pickling treatment is conducted for a time toachieve a weight reduction through said pickling treatment within arange of 0.1-20.0 g/m².
 3. The method defined in claim 1 wherein saidrecrystallization annealing is conducted in the range between therecrystallization temperature and the AC₃ transformation point of thesteel.
 4. The method defined in claim 1, including the step of coatingsaid sheet with porcelain enamel after forming said shape.